1. Field of the Invention
The invention relates to a micropattern forming method and an atomic lithography apparatus for forming a pattern on a substrate and, more particularly, to atomic lithography which freely draws a desired periodic pattern on a substrate and which has a high drawing resolution which is a diffraction limit of light or less in a technique (atomic lithography) for controlling movement of atoms or molecules with a laser and depositing the atoms or molecules on a substrate to form a microstructure.
2. Description of the Related Art
A reduction optical lithography system is popularly used in manufacturing a semiconductor integrated circuit with a pattern formed in a sub-micron region. In this method, the following processes are used. That is, a photosensitive resist agent is coated on the surface of an object on which a micropattern is to be formed, the micropattern is formed by using the reduction optical system, and the surface of the object is etched by using the photosensitive resist film as an etching mask. The unnecessary photosensitive resist mask is removed to form a desired micropattern. When the photosensitive resist film is used in this method, a small amount of impurity is diffused on the surface of the object on which the micropattern is to be formed which adversely affects the electronic characteristics of the surface. In another method of drawing a micropattern, not only simultaneous exposure but also drawing using a narrow electronic beam are performed. However, in this method, repulsion (Coulomb interaction) between electrons regulates a drawing resolution in convergence of the electron beam. Since a photosensitive resist agent is also used, there is the same problem as that in the above case.
Recently, a method using interaction between an optical standing wave and electrically neutral atoms has attracted attention (for example, see the following Non-patent Document 1 (G. Timp, et al., “Using light as a Lens for Submicron, Neutral-Atom Lithography”, Phys. Rev. Let., 69, 1635–1639, 1992)). The Non-patent Document 1 describes, as can be seen in FIG. 1 in the document, a standing wave having a diameter of about 300 μm formed by using a laser having a wavelength of 589 nm, and propagated in a direction perpendicular to the direction of an atomic beam having a low average speed of 740 m/s to draw a strip pattern on a substrate, with the results being that the line width of the strip pattern is narrowed.
Non-patent Document 2 (A. S. Bell, et al., “Atomic Lithography”, Microelectronic Engineering 41/42, 587–590, 1998) demonstrates in a configuration shown in FIG. 1A of Non-patent Document 2, a standing wave having a grid-like pattern generated by two reflecting plates and a laser having a wavelength of 425 nm, so that a grid point group has a cycle which is ⅔ the wavelength of the beam. A chromium atomic beam is generated and changed into an atomic line with a parallel moving direction by using a laser cooling method, so that a laser has the wavelength which is close to the wavelength at which resonant transition of chromium atoms is caused.
When utilizing a laser which has an uneven laser intensity distribution and a wavelength longer than the resonant transition wavelength of the atoms used, the atoms are subjected to a force such that the atoms are moved toward a region having high laser intensity in the light field. On the contrary, when a laser which has a wavelength shorter than the resonant transition wavelength is used, the atoms are subjected to a force such that the atoms are moved toward a region having small laser intensity. In the above Document 2, by using the characteristics, the pattern is formed on the silicon substrate, and a photosensitive resist agent is not formed. For this reason, the silicon substrate is not easily polluted.
FIG. 1 shows an example of a conventional method of forming a micropattern (see the following Patent Document 1 (Japanese Patent Application Laid-Open No. 2002-75825)).
FIG. 1 is a pattern diagram showing an example of conventional atomic lithography performed in a vacuum chamber 100. A material used in the lithography is put in an oven 10 to heat and evaporate the material. This evaporated gas is collimated by using two pin holes (the first pin hole is in the oven, and the second pin hole is in a collimator 2) arranged coaxially in the direction in which the evaporated atoms scatter. A group of atoms treated in this manner generate a thermal atomic beam.
Since the thermal atoms have various speeds, the atoms are moderated by the well known scattering force of the beam or sorted depending on the speeds, so that the atoms are moderated to have a speed of, e.g., 5 m/s or less.
A magneto-optical trap (MOT) captures the moderated atomic beam obtained as described above. At the same time, a laser cooling operation is performed to cool the atoms such that the kinetic energy becomes energy corresponding to a temperature of 1 mK or less. The MOT is constituted by an AntiHelmholtz coil 5 and a laser 6. The AntiHelmholtz coil 5 may be arranged either on the outside or the inside of the vacuum chamber. Further, a current is flowed into the coil such that a magnetic gradient generated by the coil is about 1 mT/cm. The MOT laser is caused to be incident in six directions (±x, ±y, ±z) from the outside of the vacuum chamber through a view port.
When the group of atoms are captured by the MOT and sufficiently cooled, irradiation of the laser used in the MOT is interrupted, and a group of cooled atoms 3 is caused to free-fall by about 10 cm according to gravitational force. To control the kinetic energy of the atoms, the distance is controlled. At this time, the cooled atoms are arranged to fall vertical in the direction of gravity.
The quadrupole magnetic field generated at this time is generated as follows. That is, four copper rods 4 (length of 10 cm) are arranged at equal intervals (10 mm) as shown in FIG. 1, and currents opposite to each other are flowed into the rods to generate a magnetic field (maximum magnetic field=15 mT) (magnetic gradient of 30 mT/cm), and the falling cooled atoms are prevented from being scattered in a lateral direction (direction in a plane vertical to the falling direction), so that a magnetic trap for increasing the density of atoms is achieved.
In addition, laser light, 8 is directed through a view port, 9 in conjunction with a solenoid coil, 7 for Zeeman Alignment Moderation. Incidence laser light, 19 and outgoing radiation laser light, 20 are also directed through view ports, 9 in proximity to the substrate, 1.
A hologram (light transmittance) is formed by calculation from a two-dimensional space pattern which is desirably drawn on the substrate 1.
In the conventional method of forming a micropattern, as described in Non-patent Document 1 and Non-patent Document 2, striped and grid-like patterns are realized.
However, since a standing wave is used in these techniques, even though the point will be modified in the future, only a simple graphic such as a polygon can be drawn.